Gaussian

A Computational Study of Molecular Conformers Jordan Haskins Andrew Fleming Amanda Petty Gino Moore

Introduction As bonds rotate, changing a molecules conformation from anti to staggered to eclipsed, the energy associated with the atoms composing that molecule changes. The purpose of this experiment is to measure the change in energy and the shielding constants for each of the hydrogen and carbon atoms in 1,2-dichloroethane.

Materials and Methods In this experiment we used Gaussian 98

to compute the energies of four different conformers of 1,2dichloroethane. These energies were then compared to the expected energies. Using Gaussian 98, we calculated the chemical shifts of the hydrogens and carbons in the conformers. Next we compared the predicted chemical shifts to the known chemical shifts given in literature.

Materials and Methods We used Microsoft Excel to perform the

calculations. Instructions for the experiment were given in a handout from Dr. Howard. Recorded energies for each conformer, along

with its isotopic shielding constant, for hydrogen and carbon where obtained from Gaussian 98.

Materials and Methods The shielding constants are: 31.8835ppm for hydrogen 187.3250ppm for carbon

Results Conformer

Calculated shielding constant (σ, ppm)

Calculated chemical shift (δcalc, ppm)

Anti

28.4778

3.4057

Staggered (hydrogens 1,2)

28.2945

3.5890

Staggered (hydrogens 3,4)

28.2340

3.6495

Eclipsed 1

28.3056

3.5779

Eclipsed 2 (Hydrogens 1,2)

28.3749

3.5086

Eclipsed 2 (Hydrogen 3,4)

27.9314

3.9521

Results Conformer

Calculated shielding constant (σ, ppm)

Calculated chemical shift (δcalc, ppm)

Anti

138.0219

49.3031

Staggered

133.2141

54.1109

Eclipsed 1

140.2585

47.0665

Eclipsed 2

133.7496

53.5754

Results The experimental chemical shifts of 1,2 dichloroethane were:

Hydrogen

Carbon

3.729 ppm

43.60 ppm

These values were given in the Spectral Database System.

Results Conformer

Calculated Experimental Chemical shift Chemical shift (δcalc ) (δexp)

Percent Error (%)

Anti

3.4057 ppm

3.729 ppm

8.67

Staggered (Hydrogens 1,2)

3.5890 ppm

3.729 ppm

3.75

Staggered (Hydrogens 3,4)

3.6495 ppm

3.729 ppm

2.13

Eclipsed 1

3.5779 ppm

3.729 ppm

4.05

Eclipsed 2 (Hydrogens 1,2)

3.5086 ppm

3.729 ppm

5.91

Eclipsed 2 (Hydrogens 3,4)

3.9521 ppm

3.729 ppm

5.98

Results Conformer

Calculated Chemical shift (δcalc )

Experimental Chemical shift (δexp)

Percent Error (%)

Anti

49.3031 ppm

43.60 ppm

13.1

Staggered

54.1109 ppm

43.60 ppm

24.1

Eclipsed 1

47.0665 ppm

43.60 ppm

8.0

Eclipsed 2

53.5754 ppm

43.60 ppm

22.9

Results Conformer

Energy (Hartrees)

Anti

-1000.04022175

Staggered

-1000.03796939

Eclipsed 1

-1000.02614119

Eclipsed 2

-1000.03303489

Discussion Based on our calculations, we found that

the anti conformer was the most stable. This is due to the fact that it is where the largest electron clouds are the farthest apart. Also, the eclisped conformer in which the chlorines overlap was the least stable. This is due to the largest electron clouds being in closest proximity.

Discussion Since rotation is rapid around the C-C

double bond, the NMR instrument recognizes only an “average” molecule. So, the chemical shifts are an average of the different conformers. Gaussian gave chemical shifts that were different than the reported experimental values: the carbon chemical shifts were too high the hydrogen chemical shifts were too low

This is due to Gaussian underestimating

the electron density near the nucleus if the chemical shift is too high and overestimating the electron density if the

Conclusion Since the model system used consisted only

of a single, motionless, gas-phase molecule, the setup wasn’t realistic. This could be remedied by having more molecules in the system so that there would be more molecular interactions. This may influence more accurate chemical shifts.

Citations Handout: A Computational Study of Molecular Conformers Spectral Database System.

http://www.aist.go.jp/RIODB/SDBS/menu-e.html Fleming, A.J.; Physical Chemistry Laboratory Notebook, 2007 p. 17-18. Haskins, J.R.; Physical Chemistry Laboratory Notebook, 2007 p. 22-23. Moore, L.G.; Physical Chemistry Laboratory Notebook, 2007 p. 18-19. Petty, A..; Physical Chemistry Laboratory Notebook, 2007 p. 24-25.